French patent document FR-A-2 585 200 proposes detecting the periodic component in the armature feed current of a D.C. motor and counting the number of periods in said periodic component in order to monitor the position of the armature and thus the position of a moving member associated therewith. The periodic component in the armature current is generated by the periodic switching of the commutator brushes. The number of armature current oscillations generated from a known reference position is therefore directly representative of the instanteous position of the motor. The principle outlined in document FR-A-2 585 200 is attractive in theory since it enables position to be controlled without requiring auxiliary sensors such as optical sensors, Hall effect sensors, or the like, which are conventionally used for monitoring the position of the motor.
In the context of the motor industry, by way of non-limiting example, armature current ripple counts can be used for monitoring the positions of moving members such as rearview mirrors, seats, steering wheels, air conditioning controls, . . . .
However, in practice it is observed that the armature current of a D.C. motor includes a large amount of interference.
Accompanying FIG. 1 shows the armature current of a D.C. motor during a start-up stage, and accompanying FIG. 2 is on a larger scale and shows the armature current of a D.C. motor while running normally. After a start-up stage during which a current surge is drawn by the armature circuit of the motor, current stabilizes in normal operation about a value which is practically constant for any given motor mechanism, with a periodic component generated by commutator switching being superposed on the substantially constant value. The period of this component is referenced T in accompanying FIGS. 1 and 2.
On examining FIGS. 1 and 2, it can be seen that the armature current also includes a large amount of random interference (harmonics and/or artifacts). If this interference is taken into account by the counting means, then the count of the number of oscillation periods in the armature current will be wrong, and thus the position will also be wrong.
An attempt has been made to eliminate the interference superposed on the armature current of a D.C. motor by means of the circuits shown in accompanying FIG. 3, where a signal representative of the armature current is taken from the terminals of a shunt resistance connected in series with the armature, is filtered by a low pass filter, and the filtered signal is applied to a trigger stage.
However, the circuit shown in FIG. 3 remains highly sensitive to harmonics and to large amplitude interference. The error rate in counting the ripples in the armature current varies between 1% and 30% depending on the state of the motor and on the electromagnetic environment.
The object of the present invention is to eliminate this drawback.